1. Field of the Invention
The present invention relates to an optical element for use in a liquid crystal display device.
2. Background Art
In recent years, reflection liquid crystal display devices requiring no backlight have begun to be used in displays for portable equipment such as mobile PCs by taking advantage of their features such as low power consumption, small thickness and lightweight.
Since, however, conventional reflection liquid crystal display devices display information by using external light, when the reflection liquid crystal display device is used in a dark environment, the display screen is also dark. In particular, in darkness, the display screen cannot be viewed at all. This renders the reflection liquid crystal display device unusable. In order to overcome the above problem, a semitransmission liquid crystal display device equipped with a light source has been developed so that, in a dark environment, the liquid crystal display device can be utilized as a transmission liquid crystal display device.
Thus, both the conventional transmission liquid crystal display device and the semitransmission liquid crystal display device require the use of a backlight as a light source. In these conventional liquid crystal display devices, the power consumption of the backlight is large. In particular, in the case of a battery-driven type, the length of usable time is limited. Therefore, features of the display for portable equipment cannot be fully utilized.
With a view to solving the above problem, a liquid crystal display device, which effectively utilizes light from a backlight, is under development. For example, Japanese Patent Laid-Open No. 98961/2002 discloses a semitransmission liquid crystal display device with high transmittance in which a scattering polarizing plate is provided on the backside of a backlight so that light, which has been emitted from the backlight and reflected from a semitransmission reflector plate, is not absorbed in a polarizing plate.
Further, Japanese Patent Laid-Open No. 275629/2000 discloses a liquid crystal display device which has been improved in utilization efficiency of light by using a cholesteric liquid crystal and a phase difference plate, i.e., a retardation film, in a black matrix.
In the liquid crystal display device disclosed in Japanese Patent Laid-Open No. 98961/2002, however, light, which has been emitted from the light source and absorbed or reflected from a light-shielding layer and the like, is disadvantageously absorbed in a linearly polarizing plate and cannot be effectively utilized. Further, in the liquid crystal display device disclosed in Japanese Patent Laid-Open No. 275629/2000, the cholesteric liquid crystal should be stacked, and, in addition, patterning is necessary. This increases the number of production process steps, leading to an increase in cost.
Specifically, in the conventional semitransmission liquid crystal display device, when a circularly polarizing plate is used as the polarizing plate on the light source side, as shown in FIG. 18, light emitted from the backlight source is passed through a linearly polarizing plate and consequently converted to linearly polarized light. The light converted to the linearly polarized light is then passed through a quarter-wave (¼λ-wave) phase difference plate and consequently converted to right-handed circularly polarized light or left-handed circularly polarized light. The light converted to the circularly polarized light is incident to a liquid crystal layer or is reflected from a reflecting electrode provided in the liquid crystal layer. In this case, the direction of circular polarization of the light reflected from the reflecting electrode becomes opposite to the direction of circular polarization of the light before incidence to the reflecting electrode. Thereafter, the reflected light is again passed through the phase difference plate and consequently is converted to a linearly polarized light of which the direction is orthogonal to the transmission axis of the linearly polarizing plate. Therefore, the linearly polarized light cannot be passed through and is disadvantageously absorbed in the linearly polarizing plate. Further, in the conventional transmission liquid crystal display device, as shown in FIG. 18, a light-shielding layer composed of a black pigment or a metal such as chromium (Cr) and CrO or the like is provided between individual colors of a color filter (for example, between B (black) and R (red), between R (red) and G (green), or between G (green) and B (black). The light-shielding layer functions to prevent the contrast ratio from being lowered as a result of leakage of light from the backlight from between the individual colors and to prevent a lowering in off resistance caused by the arrival of external light at TFT. The light-shielding layer is used for preventing the transmission of light emitted from the backlight to the viewer side. Therefore, in the light-shielding layer formed of a black pigment, disadvantageously, light introduced from the backlight is substantially entirely absorbed, and, also in the light-shielding layer composed of a metal such as chromium and CrO, the reflectance is low although a certain level of the light is reflected toward the backlight side.
Thus, the conventional liquid crystal display devices involve a problem that light supplied from the backlight to the reflecting electrode or the light-shielding plate cannot be satisfactorily reused.
Further, when the cholesteric liquid crystal and the phase difference plate are used in the black matrix, the structure of the display device is complicated. This requires many process steps, leading to a problem with a high cost.